The present invention relates to the formation of 1,3-diaryl triazenes, e.g., 1,3-diphenyltriazene, by the diazotization-coupling of primary aromatic monoamines, e.g., aniline.
2. Description of the Prior Art
1,3-Diaryl triazenes (also referred to as diazoamino aryl compounds) can be made to undergo rearrangement by heating, e.g., in the presence of a Friedel-Crafts catalyst, to form aromatic amino azo compounds. The latter are useful for the preparation of aromatic diamines by reduction with hydrogen. The diamines, in turn, are useful as intermediates in the preparation of polymers, antioxidants, etc.
U.S. Pat. No. 2,894,942 describes the formation of aromatic amino azo compounds by adding an inorganic nitrite to a mixture of an excess of a primary aryl monoamine, a mineral acid, and a Friedel-Crafts catalyst while the temperature is 0.degree.-50.degree. C., and thereafter heating up to about 40.degree.-75.degree. C. A portion of the amine is diazotized and the diazonium salt couples with unreacted amine to form the diazoamino aryl compound, which rearranges to the amino azo compound under the combined influence of the catalyst and heat.
A recently discovered process for diazotizing/coupling primary aromatic monoamines involves the reaction of the monoamine with a diluted nitrogen oxide-containing gas mixture derived from the oxidation of ammonia. One of the advantages of this process, as contrasted to the nitrite/acid diazotization method, is that no salt byproducts are formed that require troublesome salt separation and disposal procedures. In the aforesaid diazotization/coupling process, a gas mixture comprising a diluted nitrogen oxide component selected from the group consisting of nitrogen dioxide (NO.sub.2) and mixtures of nitrogen dioxide and nitric oxide (the nitrogen oxide component being represented by the formula NO.sub.x wherein x is (1 + n), n being the NO.sub.2 fraction of the nitrogen oxide component, and generally has a value of 1.1 to 2.0, preferably about from 1.3 to 1.7) is contacted with the monoamine in the liquid phase, the contact between the NO.sub.x gas and the liquid reaction mixture being interrupted while at least about 5 percent, and preferably at least about 40 percent, by weight of the monoamine remains unconsumed. Thereafter, a residual gas consisting chiefly of the diluent, usually nitrogen, present in the oxygen-containing gas used to produce the nitrogen oxide-containing gas mixture from ammonia, and containing possibly unreacted NO.sub.x and oxygen as well as water, monoamine, and 1,3-diaryl triazene in the vapor phase is separated from the triazene-containing reaction liquid. The excess monoamine present in the reaction mixture enables the initially formed diazonium nitrate to couple instantaneously with the monoamine to form the triazene and thereby prevents the build-up of uncoupled diazonium nitrate in the reaction mixture. The accumulation of the nitrate anywhere in the reaction system is undesirable because of the potential explosion hazard created thereby.